Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Particularly, light olefins are used in the manufacture of polyolefins such as polypropylene and polyethylene. Catalysts for polyethylene and polypropylene require a product that is substantially free of contaminants such as sulfur and nitrogen. When sulfur and nitrogen compounds are present in the olefin feedstock, the catalyst is rendered less effective resulting in poorer quality goods or less efficient polymerization products.
One emerging technology for the production of light olefins uses oxygenate feedstocks such as methanol, ethanol or dimethyl ether. Methanol, ethanol and dimethyl ether feedstocks are produced from synthesis gas derived from natural gas or other sources. Oxygenate feedstocks produced from this method are desirable because they contain negligible amounts of nitrogen or sulfur and result in olefin products that are less poisonous to polymerization catalysts. One embodiment of the reaction of oxygenates to olefins uses a molecular sieve catalyst, such as a SAPO catalyst, in a reactor system that has an oxygenate to olefin (“OTO”) reactor and a catalyst regenerator. The catalyst in the OTO reactor converts oxygenates to olefins and also generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Over accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. Thus, the molecular sieve catalyst is periodically recycled to the catalyst regenerator. During regeneration, the coke is removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst. The regenerated catalyst is then recycled back to the OTO reactor where it is reused to catalyze the OTO reaction.
U.S. Pat. Nos. 6,023,005 and 6,166,282, both of which are incorporated herein by reference, disclose methods of producing ethylene and propylene by catalytic conversion of oxygenate in a fluidized bed reaction process which utilizes catalyst regeneration.
U.S. Pat. Nos. 4,595,567, and 4,615,992, both of which are incorporated herein by reference, disclose general and specific regeneration devices and techniques.
The reactor system comprising an OTO reactor and a regenerator often requires the addition of heat to the reactor system. The OTO reaction is exothermic, requiring an initial heating to initiate the reaction, after which it is self-sustaining. There are also periods where the oxygenate feed must be interrupted, at which time it would be desirable to keep the reactor and regenerator hot. In addition, the initial start-up of the regenerator and heating of the catalyst also requires heat.
Conventionally, the regenerator apparatus is preheated by an auxiliary burner which burns a starting fuel such as natural gas with air to provide a heated gas that contains air and gaseous combustion products such as carbon dioxide and water (steam), to the regenerator. The auxiliary burner can be associated with the regenerator air blower that introduces the heated gas through an air inlet at the bottom of the regenerator. However, given the low heat capacity of such heated gas, resulting in part from its expansion upon heating, the heat input to the unit is limited. Particularly, the maximum amount of heat and the maximum temperature is limited. When heat is added to reactors other than OTO reactor systems, such as a fluid catalytic cracking (FCC) system, hydrocarbon feed (gas oil) to the FCC unit is burned in the regenerator to heat the catalyst. However, the gas oil feedstock of an FCC unit is contaminated with nitrogen and sulfur and would be unsuitable in an OTO process, as the gas oil would increase the levels of these contaminants in the products. Methanol is undesirable as a fuel for heating catalyst because it has a high autoignition temperature, and igniting and sustaining the burning of methanol would be difficult. The process of fluid catalytic cracking (FCC) normally circulates hot catalyst from the regenerator to the reactor to add heat to the reactor. One method of doing this combusts a fuel with the air feed to the regenerator. The FCC process normally uses fuel gas (including natural gas), or a combination of fuel gas and heavy liquid feedstock for this purpose. The fuel gas is combusted in an auxiliary burner, located after the air blower but before entering the fluidized bed of catalyst in the regenerator. The limited heat capacity of the regeneration air, resulting in part from its expansion upon heating, limits the rate at which heat is added to the regenerator through this method. It is normally desirable to add heat at a greater rate, and thus a liquid fuel, normally consisting of gas oil feedstock, is added to the fluidized catalyst zone. The catalyst has a much higher heat capacity than the combustion air, and thus liquid fuel can be added at a much greater rate in the fluidized catalyst zone than can be added to the combustion air through the auxiliary burner. The gas oil also has a relatively low autoignition temperature of 315-370° C. (600-700° F.), which aids in the initiation of combustion, as well as helping to ensure that the combustion will not be extinguished during a low temperature excursion.
In trying to adapt the FCC heating method to the MTO process, some major problems are encountered. Gas oil cannot be used as the heating fuel, because the sulfur and nitrogen introduced by the gas oil cannot be tolerated in the MTO product recovery section. The MTO feedstock, methanol, cannot be used as the heating fuel, because its autoignition temperature of 468° C. (875° F.) is so high that preheating the catalyst bed in the regenerator to a sufficient temperature to initiate the combustion of methanol is difficult. Also, there is a greater risk of extinguishing the burn from a low temperature excursion.
Accordingly, it would be desirable to provide a process for making olefins from oxygenate which has an initiation procedure which provides a high heat input to an oxygenate to olefins reactor system within a reasonable time, hours rather than days, to provide supplemental heat to the reactor, without contaminating either the OTO catalyst or the MTO products and byproducts. The present invention satisfies these and other needs.